1. Field of the Invention
The present invention relates to a contact element for the intermittent contacting of conductor tracks on a circuit board, in particular for flexible keypads or input devices. Furthermore, it relates to a method for producing flexible keypads or input devices and to uses of such contact elements.
2. Description of Related Art
Silicone keypads are also being used increasingly frequently in the automobile sector, such as for example for switching mirror adjusters, window openers, etc.
In resistance-coded circuits, it is decisive that a contact element that is of very low resistance and reliable is available. If contact elements that are of high resistance and unreliable are used, contact disturbances or even failures occur. It is likewise of great importance that high currents of, for example, 200 mA can be switched, in particular for the direct switching of motors.
In the case of such keypads, typically a flexible pad, for example of silicone, is arranged over a circuit board. Arranged on the circuit board are conductor tracks, which have interruptions at the locations to be actuated. Arranged on the underside of the flexible silicone pad, and bridging these interruptions, are contact elements, which are at a distance from the circuit board. These contact elements are typically referred to as contact pills. If the flexible pad is pressed down in the correct region, contacting takes place.
For such applications, today normally two different types of contact pills are used, both with serious disadvantages.
Carbon Pill:
Silicone material is made conductive with carbon or other electrically conducting particles, pressed into sheets and punched out, and then vulcanized together with the keypad in the compression mold.
A disadvantage of these pills is always that the contact resistance is dependent on the pressing pressure, i.e. if the key is pressed only quite lightly, there is a very high switching resistance, which can be misinterpreted. This is unacceptable in particular whenever different functionalities are to be coded in series over the same conductor path by way of usually different sized resistances, since a completely different functionality that happens to be arranged in the same conductor path is correspondingly triggered for example in the event of only slight actuation. Also, often contact resistances that are not really low (<1 ohm) are achieved.
Gold Pill:
A copper sheet is laminated on one side with a silicone layer and coated on the other side with metal and gold. Pills are then punched from the sheet and vulcanized together with the keypad in the compression mold.
A disadvantage of this pill is the high price of the material and the high reject rates in the process, and also the high susceptibility to contamination. As soon as a grain of dust gets in between the pill and the circuit board, the contact is isolated and fails on account of its rigid structure. A further disadvantage is that the gold pill only responds under certain pressing pressure and not directly on contact with the circuit board.
Metal Gauze:
DE 23 35 907, U.S. Pat. No. 5,047,602, EP 0 938 111 and others also describe the possibility of providing a woven or a nonwoven fiber structure of metal or carbon fibers or of conductively coated fibers as a contact region for a switch. A disadvantage of such solutions, entirely analogously to the particles in the carbon pill mentioned, is the fact that it always has to be ensured that the fibers are actually in sufficient contact in order to ensure the conductivity. Moreover, precisely this contacting is a property that rapidly deteriorates when a switch is intensively actuated, and the fibers have the tendency to break when they age.